Credit

Caption

Pine (Pinus sp.) needle. Polarised light micrograph of a longitudinal section through a pine needle, showing stomata (oval). Stomata are pores that are the site of gaseous exchange in the plant. The opening and closing of the stomata is controlled by semicircular guard cells (sausage-shaped). When the guard cells are turgid the stomata are open and when they are flaccid the stomata are closed. Magnification: x200 when printed 10 centimetres wide.